The present invention relates to apparatus and methods for measuring displacements of a displaceable member along a predetermined displacement path. The invention is described below particularly with respect to rotary displaceable members such as in rotary encoders, but may also be used with respect to linearly-displaceable members, such as in linear encoders.
The displacement of rotary members is usually measured by counting the number of turns or revolutions experienced by the rotary member as well as fractions thereof, which fractions determine the resolution of the measurement apparatus. Turn counting is needed for example in absolute encoders mounted on motors, e.g., as explained in U.S. Pat. No. 6,628,741 by Netzer.
Turn counting systems are used in rotary encoders to provide absolute position information of high precision even when there is an interruption of the power supply to the system and the shaft to be monitored has been turning during these power supply interruptions. Many absolute encoders use batteries in order to monitor and record the turn counting while external power is interrupted. However the use of battery has a number of drawbacks. Thus, batteries have a limited life time. Moreover, replacing a battery without losing the recorded turn counting requires special circuitry, for example a large capacitor, to back up the recorded data during the battery replacement, which circuitry results in additional cost of the encoder. Further, batteries tolerate a limited range of temperatures, and therefore where the encoders are to be used in high temperature environments, the battery cannot be placed inside the encoder.
U.S. Pat. No. 5,565,769 and U.S. Pat. No. 5,714,882 describe systems that are able to count and register the number of turns of a shaft without a battery; however, these systems are sensitive to vibrations. U.S. Pat. No. 6,628,741B1 describes apparatus to implement a turns counter without a battery by using a reed relay; however reed relays are sensitive to vibrations, have a limited life time, and can be damaged or destroyed in case of high accelerations. Another drawback is that the amount of energy produced in order to count and store the number of turns is very small, which limits the system to the use of ferroelectric memories; unfortunately, these memories are not available in small sizes, and this again limits their application in encoders.
Similar problems are involved in linear encoders for measuring linear displacements of a displaceable member.